This invention relates to the field of drying (or dehydration) of vegetable matter. Its subject is a drying process of high energy efficiency, which employs high-pressure steam.
Classical processes for the dehydration of fodder employ direct heat exchange between the moist matter and a stream of hot gas in a rotary, for example parallel-flow, drum dryer. Thermal efficiency is proportionately higher the higher the temperature of these gases.
In most cases these hot gases are produced by dilution of combustion gases produced using heating oil, coal, natural gas, biogas or straw, or else air heated in an electric battery. Depending on the temperature, the energy yield varies between 750 and 800 kcal per kg of evaporated water.
Processes based on recovery of the energy of gases produced in a rotary dryer can improve this performance.
Use is then made either of a belt dryer fed with hot air produced in an air condenser placed downstream of the rotary dryer or, when there is a liquid to be concentrated, a multiple-effect vacuum evaporator.
The specific usage can then come down to 450 kcal/kg of evaporated water.
Drying which employs steam, as applied to other materials, is not used for dehydration because of its high energy cost.
At temperatures which are usually accessible with steam, the energy consumption of the dryer as such cannot be reduced below 900 kcal per kilogram of evaporated water. However, this figure must be increased further by 15% to express the usage based on the pimary energy expended in the evaporation boiler, without prejudice to other cost factors in steam production.